Recently, an automobile is provided with an electric wave receiving device other than a car radio, e.g., an electric wave receiving device for a keyless entry system. The keyless entry system is a control device for opening or closing a door of a vehicle and includes a keyless entry receiver installed inside the vehicle and a keyless entry transmitter installed inside a key, etc. The keyless entry transmitter transmits a signal with an ID code associated with the transmitter and a modulated operational command code, when operated. When the keyless entry receiver has received the transmitted signal, the receiver demodulates the signal and determines whether the ID code is its corresponding one or not. When determined to be the corresponding one, an operation signal corresponding to the operation command code is sent to a control ECU of a corresponding component of the vehicle, thereby opening or closing the door or starting an engine. Such a keyless entry system mainly utilizes micro electric waves that are about 300 MHz. The keyless entry receiver is preferable to be installed in a position where electric waves are efficiently received. The position is remarkably in a meter unit. It is a trend that the position is a meter unit. For example, a meter unit integrated with an electric wave receiver is diffused.
In a meter unit, a meter circuit is formed on a printed circuit board, and the circuit board is housed by a meter housing. The meter circuit control indicative content indicated in an indicative portion correspondingly to actual driving state of a vehicle. Recently, the meter unit is however provided with a CPU similarly to the other electric devices, thereby accomplishing advanced indicative performance. However, the CPU, which is operated on the basis of a high-frequency clock signal, is a high-frequency signal source. Therefore, the CPU serves as a noise source in the same frequency range as the clock signal and the higher harmonics wave thereof. Accordingly, the keyless entry system is erroneously operated, and the operational frequency range of the electric wave receiver is disadvantageously narrowed.
Noise reduction technology for reducing noise in the printed board is disclosed in JP-B2-H6-34472. In this printed board, an insulative layer is formed on a board so as to cover a conductive layer, serving as a wiring pattern, excluding an earth (ground) pattern. Moreover, a shielding electrode layer, such as copper powder, is formed thereon. By virtue of electromagnetic wave shielding performance thereof, a dispensable radiation noise is reduced.
However, in the printed board, since the shielding electrode layer is formed so as to cover the conductive layer excluding the earth pattern, the insulating layer and the shielding electrode layer are formed on the substantially whole surface of the board. Therefore, this is hard to be practical.